Tank selection -- help!

[scubafanatic]

Hal,
...here's the 'formula':

'official' capacity of tank in cubic feet @ 'actual' psi in tank as measured by
'official' max psi stamped on side of tank (multiplied by) your pressure guage

...take that number and divide it by 'official' max psi stamped on side of tank...this will give you the volume of gas in your tank.

For example: I have a LP 85 cu. ft. steel tank with a maximum official fill pressure of 2640 psi , that's actually filled to 3000 psi......how much gas is really in the tank?

85 X 3000
----------------------- = 96.6 cu. ft of gas
2640


...hope that helps........ Karl

 

[SeaJay]

Steel 109's... Weird... Never heard of them.

...So I got online and checked specs... A good place for this is http://www.diverlink.com/gear/tankspecifications.htm

By "steel 109's", they must mean the Pressed Steel 120's, which actually have a capacity of 109 cuft at 2400 psi, not 120 cuft.

Aside from the fact that these steel 109's are slightly longer than your AL100 (3"-4", depending on brand name), there's a big buoyancy difference... As someone's already pointed out.

In short:

If you're currently diving a Luxfer AL100, then the rented steels are about 5.5 pounds heavier empty (AL100 = +3.5, S120 = -2) than your Luxfer AL100. To remain properly weighted, you'd need to take 5.5 or so off of your weight belt. Remember... Now you can't ditch 5.5 pounds of your weight. Consider this... Could that be a problem?

If you're currently diving a Catalina AL100, then your buoyancy is about the same as the steel tank, and little to no modification is necessary.

The big questions are:

If you're comparing a Luxfer AL100 to a Pressed Steel 120 (really, 109 cuft), then do you still carry enough weight that you can ditch and become buoyant?

If you take weight off of your weight belt because you're diving a heavier tank, then you've effectively moved your weight off of your hips and onto your back. Will that mess up your trim? (Will it make you "turtle?")

From there, make a decision on which of these tanks is right for you.

 

[LUBOLD8431]

Huh? No, they dont. If a HP 120 that has a service pressure of 3500, is filled to 2400 psi, it will have 82 cu ft of gas, not 109...
You must be a little rusty from your vacation...

Ok, just figured out you were talking about the LP tanks (service pressure of 2640)...

Carry on...

 

[WarmWaterDiver]

Hal,

There's a difference in "standard cubic feet" and "actual cubic feet". The tank ratings here are all expressed in some form of "standard cubic feet" to account for variation in physical size, nominal pressure, etc. so they're on some sort of common basis. There is more complexity in further layers here, especially if filling tanks to pressures different than the rated pressures for a particular tank, but this is the gist of the way this rating system works.

Some tanks will show two different "standard cubic feet" capacities, that are related to two different fill pressures, as is mentioned by others on this thread (109 cubic feet at 2400 PSIG pressure and also 120 cubic feet at 2640 PSIG pressure).

For the folks who stated "we're amazed your instructor didn't cover this" personally, I'm amazed no one else on this thread mentioned the basic difference between "standard" and "actual" cubic feet, as this is the basic engineering / science principle employed here. And neither in my OW, AOW, beginning Tech diving course materials, nor anywhere else on this thread are the effects of temperature on gas volume really mentioned, much less explained. I would challenge those responding here to explain the difference in available standard cubic feet of gas available from two identical tanks filled to the same pressure, but one is at 50 degrees F and the other at 100 degrees F. So if I were you I wouldn't feel belittled by some of the responses you received.

 

[perpet1]

Hal,

There's a difference in "standard cubic feet" and "actual cubic feet". The tank ratings here are all expressed in some form of "standard cubic feet" to account for variation in physical size, nominal pressure, etc. so they're on some sort of common basis. There is more complexity in further layers here, especially if filling tanks to pressures different than the rated pressures for a particular tank, but this is the gist of the way this rating system works.

Some tanks will show two different "standard cubic feet" capacities, that are related to two different fill pressures, as is mentioned by others on this thread (109 cubic feet at 2400 PSIG pressure and also 120 cubic feet at 2640 PSIG pressure).

For the folks who stated "we're amazed your instructor didn't cover this" personally, I'm amazed no one else on this thread mentioned the basic difference between "standard" and "actual" cubic feet, as this is the basic engineering / science principle employed here. And neither in my OW, AOW, beginning Tech diving course materials, nor anywhere else on this thread are the effects of temperature on gas volume really mentioned, much less explained. I would challenge those responding here to explain the difference in available standard cubic feet of gas available from two identical tanks filled to the same pressure, but one is at 50 degrees F and the other at 100 degrees F. So if I were you I wouldn't feel belittled by some of the responses you received.

Interesting challenge but not really requiired for this thread. Yes, there are other considerations like the 10% overfill on a steel tank and the fact that an AL80CF is not really 80CF but that was not the theme of the thread. Now I would like to discuss the effects of temperature but irrelevent for this thread.

Tanks are rated at a volume (CF) at a specific pressure. This is the basic concept that was missing in the initial post. There is little justification for somone who completed open water not to know this. As for the other aspects that you mention, I would expect an open water student to be able to at least discuss the effect of temperature on a tank.

 

[Hal]

Thanks to those of you who provided meaningful comments -- useful information that has helped me understand this better.

Was also starting to worry that these boards are not intended for exchange of information to increase knowledge but a place for those, ironically including the moderator, to ridicule those who are still trying to understand how things work.

Again, thanks.

Hal

 

[erubic]

"Was also starting to worry that these boards are not intended for exchange of information to increase knowledge but a place for those, ironically including the moderator, to ridicule those who are still trying to understand how things work."

-sad, isn't it. I don't think if you have anything good or positive to answer why waste the space with meaningless crap!! I thought is was a good question. Some good answers too.

 

[Groundhog246]

I don't mean to "belittle" anyone, as I do realize you don't know what you weren't taught and are in fact smart enough to ask those questions you don't know the answer to, if I came across that way, I apologize. Keep it up.
However, your not the first diver I've run across that does NOT know about cylinders, what the tank markings mean, etc. It WAS covered in our class and since the cylinder is a very essential part of your SCUBA gear, something you should know and understand. I've run across 2 people who left an LDS with rental cylinders with out of date visuals and were subsequently unable to get fills. They never learned to check.

 

[Spectre]

Ok. I know of no tank that is rated to 2450 [usually 2400 w/ the + putting it up 10% to 2640]. Nor do I know of a 3000 psi AL100 [they generally are rated to 3300].

So... An AL100 would be 100 cu ft @ 3300 psi. Therefore at 3000 psi, you'd have 90.9 cu ft.

A Steel 109 I've not heard of; but hypothetically. Most likely it'd be rated to 2400 with a 10% plus. Now the 10% plus is actually part of the rating; so it'd be 109 cuft @ 2400+10%; or 109 cuft @ 2640. So that would end up with 99 cuft @ 2400, and 101.6 cuft @ 2450.

So if you asked which had more gas, a 109 @ 2400 or a 100 @ 3300; the answer would be the AL 100. In any event however, in order to truely know we have to know exactly what the rating is on the tank [for example, Catalina AL80s are actually 77.4 cuft @ 3000 psi].

Now the other side of the coin is a LP rated tank that is filled to 3000 psi gives you more gas than it's rated for; but the key to the question is understanding that the cuft capacity is tied to a certain working pressure...

 

[cdiver2]

Thanks to those of you who provided meaningful comments -- useful information that has helped me understand this better.

Was also starting to worry that these boards are not intended for exchange of information to increase knowledge but a place for those, ironically including the moderator, to ridicule those who are still trying to understand how things work.

Again, thanks.

Hal

Dont get upset we are not all like that infact some of us that are not instructors enjoy helping new guys out. If a persone has to put someone down to show how good they are stay away from them let someone else feed there ego.